Typical integrated memory circuits include arrays of memory cells arranged in rows and columns. In many such integrated memory arrays, several redundant rows and columns are provided to be used as substitutes for defective locations in memory. When a defective location is identified, rather than treating the entire array as defective, a redundant row or column is substituted for the defective row or column. This substitution is performed by assigning the address of the defective row or column to the redundant row or column such that, when an address signal corresponding to the defective row or column is received, the redundant row or column is addressed instead.
To make substitution of the redundant row or column substantially transparent to a system employing the memory circuit, the memory circuit includes an address detection circuit. The address detection circuit monitors the row and column addresses and, when the address of a defective row or column is received, enables the redundant row or column instead.
One type of address detection circuit is a fuse-bank address detection circuit. Fuse-bank address detection circuits employ a bank of sense lines where each sense line corresponds to a bit of an address. The sense lines are programmed by blowing fuses in the sense lines in a pattern corresponding to the address of the defective row or column. Addresses are then detected by first applying a test voltage across the bank of sense lines. Then, bits of the address are applied to the sense lines. If the pattern of blown fuses corresponds exactly to the pattern of address bits, the sense lines all block current and the voltage across the bank remains high. Otherwise, at least one sense line conducts and the voltage falls. A high voltage thus indicates the applied address corresponds to a defective row or column.
An alternative address detection circuit employs antifuses in place of conventional fuses. Antifuses are capacitive-type structures that, in their unblown states, form open circuits. Antifuses may be "blown" during programming by applying a high voltage across the antifuse. The high voltage causes the capacitive-type structure to break down, forming a conductive path through the antifuse. Therefore, blown antifuses conduct and unblown antifuses do not conduct. However, due to variations among the individual antifuses, the response to the high-voltage may vary significantly across a particular group. For example, some of the antifuses may blow quickly while other, more robust antifuses may take significantly longer to blow. Consequently, more robust antifuses may be only marginally blown during programming.
Individual antifuses are programmed to form a pattern corresponding to the address of the defective row or column. Generally, the individual antifuses are read by a antifuse reading circuit which generates a digital value or signal indicating whether the antifuse is blown or unblown. The resulting pattern of digital values provides the address of the defective row or column. When a memory device is accessed, the individual antifuses are read and a resulting pattern of the antifuses is compared with an incoming address. If the addresses match, the address detection circuit generates a match signal indicating that the address programmed by the antifuses has been detected. As a result, a redundant row or column is accessed instead of the defective location.
Shown in FIG. 1 is a subsystem 100 of a memory device having a conventional antifuse reading circuit 106 that can be used to read an antifuse 130, and generate a FUSE* signal indicating the blown or unblown state of the antifuse 130. Several elements of the subsystem 100 that are related to programming the antifuse have been omitted from FIG. 1 in the interest of brevity, and are not needed in explaining the reading operation of the conventional antifuse reading circuit 106. An external voltage VCCX is applied to an external terminal 101. An internal power source 102 receives the VCCX voltage and generates an internally regulated voltage VCCR that is used by the internal circuitry of the memory device. The VCCR voltage is provided to a logic circuit 104 that generates an SV signal used during a fuse reading operation, and is also provided to a node 108 of the antifuse reading circuit 106. The internal power source 102 is designed to provide a VCCR voltage that is relatively constant over a predetermined voltage range of the VCCX voltage. Although the VCCR voltage is regulated, it will nevertheless increase when the VCCX voltage increases in the predetermined range, but not to the same degree as the VCCX voltage. The design and operation of the internal power source 102 is well known in the art.
The antifuse reading circuit 106 is enabled by an active low signal FP*. The FP* signal is generated by a control circuit (not shown) and is normally high until a fuse read operation is to be initiated. When the FP* signal goes low, a PMOS transistor 110 couples the VCCR voltage to the antifuse 130 through a PMOS transistor 114, and NMOS transistors 124, 126. A gate of the NMOS transistor 124 receives a signal DVC2E which is slightly greater than one-half of the VCCR supply, and maintains the NMOS transistor 124 in a conductive state. Similarly, a gate of the NMOS transistor 126 receives a boosted voltage VCCP that exceeds the VCCR voltage, and maintains the NMOS transistor 126 in a conductive state. Therefore, for the purposes of reading the conductive state of the antifuse 130, the NMOS transistors 124, 126 will be ON. As mentioned above, when the conductive state of the antifuse 130 is to be read, the logic circuit 104 outputs a high SV signal. The high SV signal turns ON an NMOS transistor 132, thereby coupling the other terminal of the antifuse 130 to a reference voltage, such as a ground node 134. The source of the NMOS transistor 132 is normally coupled to a large negative voltage during programming. However, as mentioned above, circuitry for performing this programming will not be shown or explained in the interest of brevity.
If the antifuse 130 is unblown and remains non-conductive, the antifuse 130 will begin charging and a voltage Vn at a node 122 will increase as the antifuse 130 continues to store charge. The voltage Vn will eventually rise above the threshold voltage of an inverter 118 and trigger the inverter 118 to output a low signal. The output of the inverter 118 is in turn inverted by the inverter 120 to produce a high FUSE* signal indicating that the antifuse 130 is unblown. The gate of a PMOS transistor 112 is also coupled to the output of the inverter 118 and is turned ON when the output signal of the inverter 118 goes low to latch the high signal at the node 122. The PMOS transistor will remain conductive even after the FP* signal returns high to turn OFF the PMOS transistor 110.
On the other hand, if the antifuse 130 is blown such that it conducts current, the node 122 is essentially coupled through the NMOS transistor 132 to the switchable ground node 134 when the FP* signal goes low, despite the VCCR voltage being applied to the node 122 through the PMOS transistors 110, 114. Since the input of the inverter 118 is coupled to the ground node 134, the inverter 118 will output a high signal, turning off the PMOS transistor 112, and the inverter 120 will output a low FUSE* signal indicating that the antifuse 130 is blown. When the FP* signal returns high, the node 122 will still be coupled to the ground terminal and thus, the FUSE* signal will remain low.
Problems with the antifuse reading circuit 106 misreading the conductive state of the antifuse 130 may arise when the antifuse 130 is only marginally blown and the VCCR voltage increases above a certain threshold level. A marginally blown antifuse has a finite resistance of approximately 10-25 kohms that adds to the overall series resistance between the node 122 and the switchable ground terminal 134. Therefore, an increasing VCCR voltage will consequently increase the voltage at the node 122. At some point, the VCCR voltage may increase the voltage at the node 122 enough to trigger the inverter 118, and produce a high FUSE* signal, although the FUSE* signal should be low. The PMOS transistor 112 will be subsequently turned ON, and latch the high signal of the node 122 even after the FP* signal returns high. As previously mentioned, although the internal voltage VCCR is regulated, and has a relatively constant voltage over a predetermined voltage range of the VCCX voltage, the VCCR voltage will nevertheless increase with VCCX throughout that range, but not to the same degree as the VCCX voltage. Thus, a marginally blown antifuse may be read correctly as being blown when the VCCR voltage is at the lower end of the voltage range, but then read incorrectly as being unblown when the VCCR voltage is at the higher end of the voltage range.
In the past, variations in the VCCR voltage have, to some extent, been compensated for by corresponding variations in the SV signal since the SV signal is generated by the logic circuit 104, which, like most of the circuitry in the memory device, is powered by the VCCR voltage. Thus, the magnitude of a high SV signal varies with the magnitude of the VCCR voltage. Increasing the voltage of the SV signal increases the gate-to-source voltage of the NMOS transistor 132, and thus correspondingly reduces the channel resistance of the NMOS transistor 132. As the resistance of the NMOS transistor 132 decreases, the relative voltage level Vn at the node 122 will also decrease and compensate for the increasing voltage across the marginally blown antifuse 130, due to the increasing VCCR supply. However, as the gate-to-source voltage of the NMOS transistor 132 continues to increase, the resistance of the NMOS transistor 132 eventually becomes negligible and will no longer be able to compensate for the increasing voltage across the marginally blown antifuse 130. As the VCCR supply continues to increase beyond this point, the voltage Vn will eventually rise above the threshold voltage of the inverter 118 and the FUSE* signal output by the antifuse reading circuit 100 will erroneously indicate that the antifuse 130 is unblown. Consequently, the resulting digital pattern of programmed antifuses will no longer correspond to the address of the defective memory location.
Therefore, there is a need for a antifuse reading circuit that can accurately determine the conductive state of an antifuse, in spite of an increasing internal voltage supply VCCR.